PR toxin is a mycotoxin isolated from Penicillium roqueforti, a fungus used in the ripening of roquefort and blue cheeses. Previous studies have shown that PR toxin can inhibit biosynthesis of protein and nucleic acids in mammalian tissues. A preliminary study by J.A. Miller at the University of Wisconsin indicates that PR toxin induces tumors in rats. Chemically, PR toxin has a bis-epoxide functionality, which is ordinarily considered to be highly reactive and to display mutagenic effects. The purpose of this work is to determine the mutagenicity of PR toxin in different microbial test systems. Saccharomyces cerevisiae, Neurospora crassa and Salmonella typhimurium are employed in a liquid suspension procedure. The effects of different pH and absence or presence of liver S-9 enzymes were also examined.